#include<stdio.h>
int main()
{
    int n = 0;
    scanf("%d",&n);
    int i = 0;
    float arr[1000];
    for(i=0;i<n;i++)
    {
        scanf("%f",&arr[i]);
        arr[i]=1/arr[i];
    }
    float sum = 0;
    for(i=0;i<n;i++)
    {
        sum = sum +arr[i];
    }
    float avg = sum/n;
    float ret = 1/avg;
    printf("%.2f",ret);
    return 0;
}